The invention relates to an energy storage device.
A conventional form of energy into which, for example, excess kinetic energy is converted is, for example, chemical energy, wherein kinetic energy is initially converted into current by which, for example, a lead battery is charged. Generated electrical current may also be stored as electrical energy, for example in capacitors, in particular in high-performance capacitors, so-called “supercaps”. A further possibility relates to storing mechanical energy as kinetic or potential energy, wherein a flywheel storage system may be cited as a variant.
Such energy storage devices are able to used, for example, in hybrid or electric vehicles. In particular high-performance capacitors are used as highly dynamic buffer stores for storing electrical energy. Expediently, in an energy storage device, two electrical machines with converters are required as in a conventional flywheel, which in each case have to be designed for the maximum power of the energy source, one for storage mode and one for recuperation. Thus, for example, the size of the device is increased and the energy density of the storage device is reduced. The level of efficiency of flywheels is substantially determined by conversion losses and by the bearing friction.
High-performance capacitors or conventional flywheels with two electrical machines as energy converters are used, for example, as highly dynamic buffer stores. A drawback with such capacitors is generally that the high dependency on temperature leads to a short service life and thus to high costs and accordingly to low performance. Capacitors are able to achieve efficiency levels of more than 95%. As regards the energy density, however, the capacitors only have values of 2-5 Wh/kg. In contrast, flywheel storage systems are associated with efficiency levels of 90-95%, with an energy density of up to 50 Wh/kg.
Due to the fact that the energy content is scaled quadratically with the rotational speed, it is desirable to achieve the highest possible rotational speeds which are substantially limited by the tearing value of the rotor material. Materials such as carbon fiber-reinforced plastics (CFRP) or glass fiber-reinforced plastics (GFRP), in spite of their low density compared to steel, in theory have a maximum energy density of 1570 kJ/kg due to their high tear resistance. Where steel is used as the material, this value is 106 kJ/kg.
The high level of efficiency of flywheels has hitherto been improved by optimizing the magnetic mounting, together with partial evacuation of the rotor.
As most flywheel storage systems operate by electricity, in order to accelerate and brake the rotor, the latest developments have to be taken into account with regard to carbon fibers in composite materials, for example. The number of rotations to be applied per minute amounts to 20-50,000 or more, for example.